Why does Blender show trees in my glass cube?
The screen space reflections in Eevee especially with highly reflective materials, for example metal with 0 roughness or glass like yours, will always show the environment like a mirror.
In Material Preview (Eevee is used, even if Cycles is set as render engine), you have a few HDRIs to choose from and if no Background is set up manually, Blender will use it's preinstalled HDRIs.

The default one used is the 3rd one.
Those are only for convenience and not necessarily used for final results.
Also the place where you set your own world environment, be it a color or HDRI, is in the World Properties:

The easiest way to set up your own HDRI, is to go in the Shading tab and do the following:

Switch the mode to World

In the Shader Editor press Shift+A to add a new Environment Texture Node.

Connect the Color output of the new Node to the Color input of the Background Node.
Now you can load a HDRI in the new node. You can find free HDRIs online (HDRIHaven for example). Beware though, they are rather big files if you want good resolution.
Lastly if you have an HDRI loaded in, you want to turn the Scene World option in the Preview Mode on, so it will use your HDRI and not the default one anymore.

Here how it can look:

mind that i did not set the cube to glass
In case you just want a color, skip all the steps and directly in the World Properties the color field above the Strength can be used to change and adjust the color used for your scene. I'd advice though against using the pure color, as it really is nothing else then making the entire scene be in that one color.
Here how that can look:

There are a lot tutorials out there, worth watching, as there are so many features, that it can be overwhelming at the start.
Happy Blending